Alzheimer's disease is characterized by the abnormal deposition of β-amyloid (Aβ) in the brain in the form of extra-cellular plaques and intra-cellular neurofibrillary tangles. The rate of amyloid accumulation is a combination of the rates of Aβ formation, aggregation, and egress from the brain. It is generally accepted that the main constituent of amyloid plaques is the 4 kD amyloid protein (βA4, also referred to as Aβ, β-protein and βAP) which is a proteolytic product of a precursor protein of much larger size.
Amyloid precursor protein (APP) is a 695-770 amino acid glycoprotein, expressed in the neurons and glial cells in peripheral tissues. APP has a receptor-like structure with a large ectodomain, a membrane spanning region, and a short cytoplasmic tail. Aβ is a 39-42 amino acid peptide, constitutes part of the ectodomain of APP, and extends partly to the transmembrane domain of APP.
At least two secretory mechanisms exist which release APP from the membrane and generate soluble, truncated forms of APP (sAPP). Proteases that release APP and its fragments from the membrane are termed “secretases.” Most sAPP is released by a putative α-secretase that cleaves within the Aβ protein to release sAPPα and precludes the release of intact Aβ. A smaller portion of sAPP is released by a β-secretase that cleaves near the NH2-terminus of APP and produces COOH-terminal fragments (CTFs) which contain the complete Aβ domain.
Thus, the activity of β-secretase or β-site amyloid precursor protein-cleaving enzyme (“BACE”) leads to the abnormal cleavage of APP, production of Aβ, and accumulation of β-amyloid plaques in the brain, which is characteristic of Alzheimer's disease. In addition, the processing of APP by β-secretase is thought to be the rate-determining step in Aβ production. Therefore, therapeutic agents that can inhibit BACE may be useful for the treatment of Alzheimer's disease.
The compounds of the present invention may be useful for treating Alzheimer's disease by inhibiting the activity of the BACE, thus preventing or reducing the rate of formation of insoluble Aβ.